The present invention relates to the field of molding methods and molding apparatus. More particularly, the invention relates to a method for the spin, or centrifugal, casting of a precisely configured article such as a contact lens or other opthalmic device, disc valve, etc., from an initially liquid polymerizable mixture.
In accordance with known procedures for spin casting a contact lens such as that disclosed in U.S. Pat. No. 3,660,545, a liquid polymerizable material is introduced into a mold having a cylindrical wall and an exposed concaved bottom surface and the mold is caused to rotate about its vertical axis at a rotational speed and under polymerization conditions sufficient to create a centrifugal force which causes a radially outward displacement of the contents in the mold. By maintaining the rotating mold under predetermined conditions, the outwardly displaced polymerizable material is caused to polymerize to a solid polymeric contact lens. The resulting lens is characterized by a convex optical surface which corresponds to the concave surface of the mold and a concave optical surface whose geometric configuration has been precisely defined, to a significant degree, by the centrifugal force(s) employed during the polymerization cycle.
In one variation of this spin casting method, a plurality of individual molds each of which has been dosed with a predetermined quantity of polymerizable material is arranged in a carousel with each mold being individually rotated on its own separate vertical axis as disclosed in aforesaid U.S. Pat. No. 3,660,545.
In yet another variation of the foregoing spin casting procedure, a plurality of individual molds, each containing a precisely measured quantity of polymerizable material, is arranged in a vertically disposed rotatable polymerization tube adapted to receive the molds at its upper end. As the molds which are seated one on top of the other move downwardly through the tube due to their own weight, they pass while spinning through a zone maintained under polymerization conditions and emerge from the bottom of the tube with the lens in each mold fully formed.
These known methods of spin casting contact lenses have worked with satisfactory results employing the polymerizable materials disclosed in U.S. Pat. Nos. 2,976,576 and 3,220,960. These materials upon undergoing polymerization provide sparingly cross-linked water-absorptive polymeric hydrogels, e.g., poly(2-hydroxyethyl methacrylate) or "HEMA". When contact lenses fashioned from such hydrogels contain a high water content, they exhibit a high oxygen permeability and are therefore comfortable to wear.
However, known spin casting techniques do not provide a satisfactory lens product when the polymerizable material contains a component which has a comparatively low boiling point, which is to say, is fairly volatile under the conditions of the spin casting process. Because loss of even minute quantities of the volatile component(s) of a polymerizable material can result in unacceptable deviations from the predetermined geometric configuration of the final molded article and/or can result in off-specification changes in its composition, present spin casting procedures are unsuitable for manufacturing precisely configured articles from the relatively volatile polymerizable materials described, for example, in U.S. Pat. No. 4,440,918, the contents of which are incorporated by reference herein.
U.S. Pat. No. 4,440,918 discloses the manufacture of opthalmic devices including a contact lens, from a polymer formed from a polymerizable material containing a telechelic perfluoropolyether, a compatible comonomer and optionally, a free radical initiator or photoinitiator. The representative compatible comonomers include such relatively volatile materials as methyl and ethyl acrylate, methyl and ethylmethacrylate, and the like. Similarly, the photoinitiators and free radical initiator which may be present in the polymerizable material are also fairly volatile and include ketones such as acetophenone, 2,2-diethoxyacteophenone, etc.
Under the mildly exothermic conditions of polymerization, sufficient quantities of such volatile component(s) will be given off in the known spin casting procedures described above as to result in an unacceptable article. The problems owing to such volatilization are further compounded due to the fact that the volatilized materials frequently result in malfunctioning of the spin casting apparatus and usually being toxic in nature, pose an environmental risk. Previous attempts to overcome these problems involve additional complications rendering them impractical. One approach involves saturating the polymerization environment with the volatile component. This poses physical problems of processing in a closed system containing corrosive and/or flammable and/or toxic vapors. The existence of vapor concentration gradients within this larger closed system volume can also create problems of inconsistency.
Another approach to overcome the problems referred to, namely, doping the polymerization reaction mixture with an additional amount of volatile component(s), requires that the quantity to be added be accurately determined. At most, one can only approximate this amount and any change in the conditions from those assumed in determining the amount to be added will result in further deficiency or excess of the volatile component(s).
Commonly assigned, copending Rawlings U.S. patent application Ser. No. 690,794, filed Jan. 11, 1985 describes yet another solution to the problem posed by the volatilization of polymerization reaction component(s). A mold is described therein which possesses gas-tight sealing means which minimizes the loss of volatile component(s) from the liquid polymerizable material. However, even with this mold, there will be some loss of volatile material into the open space between the upper surface of the polymerizable mixture and the lower surface of the mold sealing means. As small as this loss may be, in come cases it may be sufficient to result in a spincast article which is of less than optimum quality.